Sulfur dioxide is commonly present in effluent gas streams from a variety of commercial sources such as, for example, stack gases from coal fired power plants, industrial boilers, metallurgical roasting operations and as tail gas streams from Claus sulfur plants, refineries and chemical plants.
A wide variety of absorption processes have been proposed which utilize absorbents including, for example, lime, limestone, sodium hydroxide, soda ash, sodium sulfite and various amines.
One particularly useful process described in U.S. Pat. No. 5,019,361, issued May 28, 1991, discloses the use of an aqueous absorbing medium containing a water soluble half-salt of a diamine, wherein the diamine in free base form has a molecular weight of less than about 300 and wherein the diamine in half-salt form has a pKa value for the free nitrogen atom of from about 4.5 to 7.3. The process disclosed by the patentees has been useful because the absorbent is regenerable, thereby, permitting cyclic operation. In addition, because the absorbent comprises a half-salt of a diamine, i.e., one amine group is in salt form, the absorbent is substantially nonvolatile and accordingly absorbent losses, e.g., during absorption and regeneration, can be minimized.
The current trend in environmental regulations has been to limit the amount of sulfur dioxide emissions to the atmosphere to lower levels than have been previously permitted. As a result, improved absorption processes and improved absorbents are desired which can provide increased capacity for sulfur dioxide and which can provide reduced levels of sulfur dioxide in the effluent gases emitted to the atmosphere. Moreover, it is further desired that the absorbents used in such processes be easily regenerable and have low volatility in order to reduce solvent losses.